Rule #1 --- Prewash flannel! Almost all shrink a lot!
Rule #2 --- I would advise not piecing with small pieces because it can ravel.
When I use it, I make large blocks, 10"X10" or 12"X12" and back with a plain piece of flannel... no batting. They make soft and cuddly little blankies. I've made many dozens of them over the years for donations to Crisis Pregnancy Centers.